The invention relates to a method and apparatus for avoiding potential accidents in water-cooled nuclear reactors.
In the case of coolant loss reactor accidents, relatively large quantities of hydrogen are liberated by radiolytic decomposition of the water and by metal-water reactions. After the start of emergency cooling, hydrogen is only produced by radiolysis. This process proceeds relatively slowly, with the result that the hydrogen concentration in a pressurized water reactor can only reach the dangerous explosion limit of 4 vol % after prolonged periods.
Various methods have already been disclosed to prevent hydrogen and air forming an explosive mixture in the dangerous concentration limits of 4 vol % to 75 vol %, said methods supposedly preventing uncontrolled hydrogen combustion
Thus, boiling water reactors, in which the lower H.sub.2 explosion limit is reached after a significantly shorter time than in the case of pressurized water reactors in the event of coolant loss, are equipped with nitrogen inerting systems. Using these systems, the air present in the containment is replaced by nitrogen to such an extent that the residual oxygen concentration is below the limiting value at which a hydrogen/oxygen explosion (oxyhydrogen explosion) is possible. However, the inerting makes normal reactor operation more complicated.
Another possibility for avoiding an explosive mixture in a reactor containment has been disclosed in U.S. Pat. No. 4,139,603. In this disclosure, a recombination facility in a reactor containment for combining hydrogen and oxygen to form water is described. The essential element of the recombination facility is a bundle of electrically heated rods by means of which the gases flowing therethrough are heated to at least about 620.degree. C. The disadvantage of this facility is to be seen in the fact that a supply of electrical energy has to be ensured in all cases.
German Offenlegungsschrift 2,922,717 (corresponds to EP-B1 0 019 907) describes a method for the recombination of hydrogen enclosed in the containment vessel of a nuclear reactor installation, utilizing a powder which reduces the hydrogen being introduced into the containment vessel using an inert gas. A catalyst which is known per se is used as the powder, with the catalyst powder together with the inert gas being blown predominantly into the upper region of the containment vessel from a storage tank arranged outside the containment vessel.
In addition to the uncertainty of the effectiveness of this catalyst in a reactor atmosphere following an accident, the settling of the fine catalyst powder blown in (Al.sub.2 O.sub.3 powder with a particle size of 20 to 60 .mu.m) in the entire containment must be regarded as a disadvantage.
In catalytic recombination, the originally present oxygen concentration of the containment air is not reduced since the radiolytically produced hydrogen only combines with the corresponding amount of radiolytically liberated oxygen.
The introduction of an easily reducible metal oxide powder for recombining the hydrogen is also known. In this case, however, not even the radiolytically liberated oxygen is recombined. The oxygen partial pressure is further increased by the radiolysis. In addition, the settling of the fine powder particles in the entire interior region of the containment is considered disadvantageous here as well.
Dangerous hydrogen concentrations in part regions of the containment can be prevented by thorough mixing of the containment atmosphere, e.g. by means of blowers and air circulation systems. However, this does not result in inerting.
The known methods aim almost exclusively to reduce the hydrogen concentration. Although, in the catalytic recombination methods, some of the oxygen is also bound, the oxygen partial pressure, during the reaction with radiolytically liberated hydrogen, does not fall below the partial pressure originally present in the containment air.
In the case of the inerting of boiling water reactors, this is achieved by a so-called preinerting. In preinerting, the containment is flushed and filled with nitrogen upon starting or restarting the plant until the Oz concentration has fallen to the specified Oz concentration.